Examples of passive lift devices mounted to the torso of a person configured to support the weight of the arm can be found in U.S. Pat. No. 9,205,017 B2 and U.S. Patent Application Publication No. 2014/0158839 A1. Such devices are seen to be limited because they are not able to automatically cut or substantially reduce their assistance when the user intends to rest his/her upper arm at his/her sides, or pick a tool from his/her tool belt. Such devices do not provide a sustained range of position where support torque automatically reduces to zero. Except for a few points of position, these devices will always be applying lifting forces to a user's upper arm, potentially inhibiting motion and creating discomfort during non-working postures when assist is not desired.
In general, passive support devices that are configured to assist a person supporting the weight of a tool are known in the art. Typical passive devices are configured to compensate for gravity under a range of positions using a combination of structural elements, springs, cables and pulleys. The configuration of these devices provides for gravity compensation within a limited range of motion. Additionally, these devices do not allow for a substantially zero torque to be provided. Examples of passive lift assist devices can be found in U.S. Pat. Nos. 6,821,259 B2 and 7,325,777. Such devices are seen to be fairly limited in terms of functionality, as their base must be repositioned every time a user moves to a new location. Examples of a passive lift assist device mounted to the torso of a person to support the weight of a tool include U.S. Pat. No. 7,618,016 B2 and U.S. Patent Application Publication No. US 2015/001269 A1. Such devices are seen to be fairly limited because of a bulky frame that does not accurately follow the motions of the user.